This application relates to a method for forming a stacked structure made of an insulating layer and an organic semiconductor layer, and also to an organic field effect transistor and a method for making same.
Existing field effect transistors (FET) including thin film transistors (TFT) that have been employed in various types of electronic devices are constituted, for example, of a channel forming region and source/drain electrodes formed in a silicon semiconductor substrate or a silicon semiconductor material layer, a gate insulating layer formed on the surface of the silicon semiconductor substrate or silicon semiconductor material layer and formed of SiO2, and a gate electrode provided in face-to-face relation with the channel forming region through a gate insulating layer. It will be noted that FET having such a structure as mentioned above is called “top gate FET” for convenience's sake. Alternatively, such transistors may be constituted of a gate electrode formed on a support, a gate insulating layer made of SiO2 and formed on the support including the gate electrode, and a channel forming region and source/drain electrodes formed over the gate insulating layer. It is to be noted that the FET having such a structure as mentioned above is called “bottom gate FET” for convenience's sake. For the manufacture of these field effect transistors having the structures mentioned above, very expensive semiconductor manufacturing apparatuses have been in use and thus, there is a strong demand for reducing manufacturing costs.
To meet the demand, attention has been drawn to studies and developments of organic FET's wherein organic semiconductor materials that are capable of making FET based on a method using no vacuum technique, such as spin coating, printing or spraying are used to form a channel forming region consisting of an organic semiconductor layer.
By the way, organic FET's are required as being capable of high speed operation because it is required that they be assembled in many electronic devices including display devices. For instance, organic FET should be ones wherein video signals are converted to data as needed and are capable of performing on/off switching operations at high speed.
For instance, with bottom gate organic FET's, a gate insulating layer made of SiO2 has been conventionally used. In order to improve the performance of a bottom gate organic FET or to improve the crystallinity of an organic semiconductor layer constituting a channel forming region, the surface of the gate insulating layer is treated with a surface treating agent, typical of which is a silane coupling agent, followed by forming a channel forming region by use of an organic semiconductor material (see, for example, IEEE Electron Letters, Vol. 18, No. 12, p. 606 (1997), written by Y. Y. Lin et at.).
For the treatment with a surface treating agent, such as a silane coupling agent, in bottom gate organic FET's, it is necessary that a gate insulating layer be formed, after which the gate insulating layer is subjected, for example, to ashing to permit OH groups to be exposed throughout the surface of the gate insulating film. Consequently, there arises a problem in that the degree of freedom in selection of the type of material for a gate insulating layer and a formation process of the gate insulating layer is low. More particularly, a problem is involved in that it is difficult, for example, to use a material of the type which is able to form a gate insulating layer by a coating procedure.
With top gate organic FET's, a channel forming region made of an organic semiconductor layer is formed on a support. In this case, in order to improve the crystallinity of the organic semiconductor layer forming the channel forming region, it is necessary to treat the surface of the support with a surface treating agent, typically a silane coupling agent. Hence, it is required that the support be uniformly exposed with OH groups on the surfaces thereof, with the attendant problem that the degree of freedom in selection of the type of material for the support and a manufacturing process of the support is low.
Accordingly, there is a need to provide a method for forming a stacked structure made of an insulating layer and an organic semiconductor layer wherein the insulating layer is capable of improving the crystallinity of the organic semiconductor layer. A need also exists to provide a method for making an organic field effect transistor which is able to improve the crystallinity of an organic semiconductor layer constituting a channel forming region and produce an organic field effect transistor having a high performance. There is also a need to provide an organic field effect transistor obtained by the method mentioned above.